1. Field of the Invention
The invention relates generally to earth-boring bits. More particularly, the invention relates to maintaining lubrication of rolling cone cutters that are mounted on journal pins. Still more particularly, the invention relates to apparatus for maintaining an appropriate thickness of lubrication between opposing surfaces that rotate relative to one another so as to extend bearing and bit life.
2. Description of the Related Art
An earth-boring drill bit is mounted on the lower end of a drill string and is rotated by revolving the drill string. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. An earth-boring bit comprises one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits comprise a bit body with a plurality of journal segment legs. The cones are mounted on bearing pin shafts (also called journal shafts or journal pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, cutter elements or teeth that extend from the cone cutters remove chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location which, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes—which in oil and gas well drilling may be miles long—must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. The amount of time required to make a round trip for replacing a bit is essentially lost time and lost productivity from drilling operations. It is therefore advantageous to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rates of penetration (ROP) so as to minimize the number of “trips” and the associated lost productivity.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are subjected to high drilling loads, high hydrostatic pressures, and high temperatures. Conventional rolling cone bits comprise lubricant systems within their journal segments for communicating lubricant from a reservoir in the bit to the narrow space—or journal gap—which exists between the journal pin and cone cutter. Seals are provided in the journal gap to prevent lubricant from escaping from around the bearing surfaces and also to prevent the cutting-laden, abrasive drilling fluid that is present in the borehole from entering the gap. Maintaining adequate lubrication of the bearings is thus critical to maintaining the life of the cone cutter assembly and of the bit. Consequently, the frequency with which the bit must otherwise be replaced due to bit failure or loss of an acceptable ROP may be reduced by maintaining proper lubrication of the cone cutters.
Thus, the embodiments of the present invention are directed toward methods and apparatus for maintaining lubrication of rolling cone cutters that are mounted on journal pins that seek to overcome certain limitations of the prior art.